This disclosure relates to the crystalline fumarate salt of (S) 3 4 difluoro 2 (2 fluoro 4 iodophenylamino)phenyl 3 hydroxy 3 (piperidin 2 yl) azetidin l yl methanone. The disclosure also relates to pharmaceutical compositions comprising the crystalline fumarate salt of (S) 3 4 difluoro 2 (2 fluoro 4 iodophenylamino)phenyl 3 hydroxy 3 (piperidin 2 yl) azetidin l yl methanone. The disclosure also relates to methods of treating cancers comprising administering to a patient in need thereof the crystalline fumarate salt of (S) 3 4 difluoro 2 (2 fluoro 4 iodophenylamino)phenyl 3 hydroxy 3 (piperidin 2 yl) azetidin 1 yl methanone.